Stephen S. Tompkins

Prediction of Coefficients of Thermal Expansion for Unidirectional Composites

Several analyses for predicting the longitudinal, α1, and transverse, α2, coefficients of thermal expansion of unidirectional composites were compared with each other, and with experimental data on different graphite fiber reinforced resin, metal, and ceramic matrix composites. Analytical and numerical analyses that accurately accounted for Poisson re straining effects in the transverse direction were in consistently better agreement with ex perimental data for α2 than the less rigorous analyses. All of the analyses predicted similar values of α1, and were in good agreement with the experimental data. A sensitivity analysis was conducted to determine the relative influence of constituent properties on the predicted values of α1 and α2. As would be expected, the prediction of α1 was most sensi tive to longitudinal fiber properties and the prediction of α2 was most sensitive to matrix properties.

Prediction of thermal cycling induced matrix cracking

Thermal fatigue has been observed to cause matrix cracking in laminated composite materials. A method is presented to predict transverse matrix cracks in a composite laminate subjected to cyclic thermal load. Shear lag stress approximations and a simple energy-based failure criteria are used to predict crack density as a function of temperature. Prediction of crack density as a function of thermal cycling is accomplished by assuming that fatigue degrades the materials inherent resistance to cracking. The method is implemented as a computer program. Simple experiments provide data on progressive cracking of a laminate with decreasing temperature, and on cracking induced by thermal cycling. Correlation of the analytical predictions to the data is very good. A parametric study using the analytical method is presented which provides insight into material behavior under cyclical thermal loads.

Electron radiation effects on the thermal expansion of graphite/resin composites

The effects of 1 MeV electron radiation on the thermal expansion characteristics of two graphite reinforced resin matrix composite systems were studied. Specimens of both graphite/epoxy (T300/5208) and graphite/polyimide (C6000/PMR15) were irradiated to a total dose of 6 x 10/sup 9/ rads at two different rates. Unidirectional, cross-ply, and quasi-isotropic laminate configurations were examined. Thermal expansion measurements were made over the temperature range of -250/sup 0/F to +280/sup 0/F with a laser interferometer. Dynamic mechanical analyses (DMA) were performed to study changes in resin chemistry. Thermal expansion results indicate that radiation did produce permanent residual strains of up to -70 x 10/sup -6/ for the graphite/epoxy when exposed to temperatures up to +280/sup 0/F. However, no permanent changes in the coefficient of thermal expansion (CTE) were observed. No permanent residual strains or changes in the CTE attributable to radiation were observed for the graphite/polyimide specimens. No significant effects of radiation dose rate on the thermal expansion of either composite system were observed. DMA results indicate that electron radiation caused chemical changes in the epoxy matrix. These changes resulted in a lower glass transition temperature and broader rubbery region which extended into the temperature range of the thermal expansion tests.

Thermal Expansion Measurements of Metal Matrix Composites

The laser-interferometric-dilatometer system currently operational at NASA-Langley is described. The system, designed to characterize metal matrix composites, features high precision, automated data acquisition, and the ability to test a wide variety of specimen geometries over temperature ranges within 80-422 K. The paper presents typical thermal-expansion measurement data for a Gr/Al rod; Gr/Al and Gr/Mg unidirectional laminates; and a Gr/Mg (+ or -8)s laminate.

Composite materials for precision space reflector panels

One of the critical technology needs of large precision reflectors for future astrophysical and optical communications satellites lies in the area of structural materials. Results from a materials research and development program at NASA Langley Research Center to provide materials for these reflector applications are discussed. Advanced materials that meet the reflector panel requirements are identified and thermal, mechanical and durability properties of candidate materials after exposure to simulated space environments are compared. Results from analytical studies to define material properties that control laminate properties and reflector deformation are discussed. A parabolic, graphite-phenolic honeycomb composite panel having a surface accuracy of 70.8 microinches RMS and an areal weight of 1.17 lbm/ft2 was fabricated with T50/ERL1962 facesheets, a PAEI thermoplastic surface film, and Al and SiOx coatings.

Development of composite materials for spacecraft precision-reflector panels

One of the critical technology needs for large precision reflectors required for future astrophysics and optical communications is in the area of structural materials. Therefore, a major area of the Precision Segmented Reflector Program at NASA is to develop light-weight composite reflector panels with durable, space environmentally stable materials which maintain both surface figure and required surface accuracy necessary for space telescope applications. Results from the materials research and development program at NASA Langley Research Center are discussed. Advanced materials that meet the reflector panel requirements are identified. Thermal, mechanical and durability properties of candidate materials after exposure to simulated space environments are compared to the baseline material.

Techniques for Measurement of the Thermal Expansion of Advanced Composite Materials

Techniques available to measure small thermal displacements in flat laminates and structural tubular elements of advanced composite materials are described. Emphasis is placed on laser interferometry and the laser interferometric dilatometer system used at the National Aeronautics and Space Administration (NASA) Langley Research Center. Thermal expansion data are presented for graphite-fiber reinforced 6061 and 2024 aluminum laminates and for graphite fiber reinforced AZ91 C and QH21 A magnesium laminates before and after processing to minimize or eliminate thermal strain hysteresis. Data are also presented on the effects of reinforcement volume content on thermal expansion of silicon-carbide whisker and particulate reinforced aluminum.

Influence of surface and environmental thermal properties on moisture in composites

Abstract An analytical, parametric study of the influence of surface and environmental thermal properties on the moisture absorption in fibre-reinforced polymeric-resin composite materials, subjected to convection and solar radiation, has been made. Predicted moisture contents, based on the conditions at the heated surface and in the ambient air, were compared for both short time and long time exposures over a wide range of values for emittance, solar absorptance, convective heat transfer coefficient, solar radiation and orientation of the surface with respect to the sun. The calculations showed that absorptance and the heat transfer coefficient have significant effects on the moisture content.